Question

Explain the osmotic pressure of a solution with the help of a thistle tube.

Answer 1

1. Osmosis can be demonstrated with the following experimental set up in which a semipermeable membrane is firmly fastened across the mouth of thistle tube. The solution of interest is placed inside an inverted thistle tube. This part of the tube and the membrane are then immersed in a container of pure water.
   
   Osmosis and osmotic pressure
2. As a result of osmosis, some of the solvent passes through the membrane into the solution. It causes the liquid level in the tube to rise. The liquid column in the tube creates hydrostatic pressure that pushes the solvent back through the membrane into the container. The column of liquid in the tube continues to rise and eventually stops rising. At this stage hydrostatic pressure developed is sufficient to force solvent molecules back through the membrane into the container at the same rate they enter the solution.
3. Thus, an equilibrium is established where rates of forward and reverse passages are equal. The height of liquid column in the tube remains constant. This implies that the hydrostatic pressure has stopped osmosis.
4. The hydrostatic pressure that stops osmosis is an osmotic pressure (π) of the solution. The hydrostatic pressure is equal to hρg, where, h is the height of the liquid column in the tube, ρ is density of solution and g is acceleration due to gravity.